Variable air volume ventilating systems were developed in response to the energy crisis experienced in the early years of the 1970 decade. While such systems reduced energy usage, they did so at the expense of comfort. For example, to obtain occupant comfort in a defined closed space, cooling air from a variable air volume terminal should be uniformly mixed with the air in the space receiving the air. The mixing requirements for these sources involves the temperature of the primary cooling air, the shape of the defined space, the velocity of the primary cooling airstream, and the diffuser design which disburses the air into the space.
When cooling demand is high in the space, the cooling air velocity from the diffuser must necessarily be high. The pressure of this high velocity air is low, causing it to cling to the ceiling of the space. As the cooling air continues to travel from the diffuser, warm air which has migrated to the ceiling within the space is drawn in and mixed with the cold airstream. The cooling air slows down and moves away from the ceiling. Since the cooling air is now mixed with warm air, the temperature is much closer to the actual room temperature. This air drops toward the bottom of the room in a broad, gentle pattern. The diffuser enables a uniform distribution of temperature and air flow to be maintained in the space.
Conversely, when the cooling load is low, the velocity of the air leaving the diffuser is low. Low velocity air does not cling to the ceiling or draw in warm air. This cold air drops in a column between the diffuser and the floor of the space. This phenomena is called "dumping," and produces a very spotty undesirable temperature and air flow envelope within the space. When the primary air temperature is low, the above described problem becomes exaggerated.
One attempted solution to correct the poor air flow envelope during low cooling loads involves raising the primary cooling air temperature. Such a technique enables a higher volume of air to be delivered to satisfy the small cooling load requirement. Thus, the higher volume of air helps enable a more uniform distribution thereof; thereby aiding occupant comfort. While this technique was acceptable for some applications, it has not proven entirely satisfactory when the primary air is used for humidity control and when the higher primary air temperature is unable to satisfy all the cooling demands (different spaces) within a given building.
Another attempted solution to correct the poor air flow envelope flowing into a space during low cooling loads, has involved using an induction type variable air volume type terminal. Such induction type terminals mix ceiling return air with primary cooling air at an air valve opening in the primary airstream at its point of highest velocity. This mixing raises the temperature of the air leaving the diffuser, but the diffuser velocity is slightly lowered. In this regard, the air drops from the diffuser, because the volume is too low to produce adequate draw to the ceiling of the space. The temperature envelope does drop downwardly from the ceiling, but the air is not as cold. Thus, the air flow pattern, overall, is not entirely satisfactory.
A further attempt at improving the temperature control and air flow distribution, has involved the use of modulating diffusers. With this mechanical device, a high velocity airstream is maintained from the diffuser to produce an air flow pattern, which extends along, and in relatively close proximity to, the ceiling of the space. The distance of path of travel of the air discharged from the diffuser is a function of the air volume. When the cooling load is low, ceiling air is mixed with primary cooling air, but the air flow pattern is reduced.
In order to improve the air flow envelopes and to obtain satisfactory temperatures of the air entering the space, a terminal fan has been utilized to draw air from the ceiling and to mix the return air from the space, with the primary cooling air. The terminal fans either have been mounted in parallel with the primary air discharge, or have been mounted in series with the primary air discharge.
The parallel fan terminals were developed for variable air volume air ventilation systems where humidity control requirements prevented the raising of the primary cooling air temperature. The parallel fan terminals mix return air from a ceiling return, with air from a primary cooling air valve at the outlet of the terminal fan. A parallel terminal fan arrangement of an improved design is shown and described in the foregoing mentioned parent patent application.
Series fan terminals mix return air from the ceiling return, with air from the primary air valve, at the inlet to the terminal fan. The conventional series fan runs continuously to provide an almost constant volume of air to the space. The primary air valve is regulated to provide the required volume of primary cooling air through the series fan to the space. The remaining air delivered by the fan is drawn from the ceiling return. In this regard, the air flow and temperature envelopes are maintained in a relatively constant manner.
While the conventional series terminal fan units have provided relatively constant air flow and temperature envelopes, the series fan had to be sufficiently large in size and capacity, to handle peak cooling loads, because the primary cooling air passes through the series fan. In this regard, the prior known series fan has operated at a peak cooling load at all times, and in this regard, has operated at or near its maximum fan speed. Moreover, since the diffuser also has functioned continuously at or near its peak output, unwanted special sound absorbing and air duct hardware has been required.
In addition to these problems, the series fan terminal has been expensive to operate, because the fan must operate continuously at maximum air volume. These economic factors use up most of the energy consumption savings that justify a variable air volume system. Moreover, the conventional series fan is large in size and capacity, and thus is undesirably noisy to operate. Thus, oversized fans are usually employed, so that they can be operated at a speed, which is substantially less than the maximum speed, for noise reduction purposes However, the larger sized fan is thus still more expensive for the initial installation costs.
In a typical building provided with a series fan variable air volume system, the primary air fan is smaller and less expensive for the building owner to operate. On the other hand, a tenant in the building is typically required to purchase the large series fan terminals for their leased space, and the tenant is required to pay for the energy required to operate the series fans. Thus, when such a prior known system is less expensive for the building owner to purchase and to use, the cost to the tenant to install and to use a series terminal, is relatively and significantly higher than a comparable parallel fan terminal.
Thus, it would be highly desirable to have a series fan system, which employs the desirable smaller sized primary air fan, and yet employ series fan terminals, which are substantially less expensive to install and to use. Also, it would be highly desirable to have such a new and improved variable air volume series fan terminal, which would be relatively inexpensive to operate and install, and which would help to maintain a relatively constant temperature and air flow envelope that would not substantially cause discomfort to the occupants of an air controlled space. Also, such a system should not require or at least greatly reduce the special sound absorbing and air duct hardware required for the system, and should not cause unwanted and undesirable noise problems for the space occupants.